text: Gas electron diffraction (GED) can be used to determine the geometry of molecules in gases. A gas carrying the molecules is exposed to the electron beam, which is diffracted by the molecules. Since the molecules are randomly oriented, the resulting diffraction pattern consists of broad concentric rings, see Figure 24. The diffraction intensity is a sum of several components such as background, atomic intensity or molecular intensity.
  In GED the diffraction intensities at a particular diffraction angle  is described via a scattering variable defined asThe total intensity is then given as a sum of partial contributions:where  results from scattering by individual atoms,  by pairs of atoms and  by atom triplets. Intensity  corresponds to the background which, unlike the previous contributions, must be determined experimentally. The intensity of atomic scattering  is defined aswhere ,  is the distance between the scattering object detector,  is the intensity of the primary electron beam and  is the scattering amplitude of the atom  of the molecular structure in the experiment.  is the main contribution and easily obtained for known gas composition. Note that the vector  used here is not the same as the excitation error used in other areas of diffraction, see earlier.
  The most valuable information is carried by the intensity of molecular scattering , as it contains information about the distance between all pairs of atoms in the molecule. It is given bywhere  is the distance between two atoms,  is the mean square amplitude of vibration between the two atoms, similar to a Debye–Waller factor,  is the anharmonicity constant and  a phase factor which is important for atomic pairs with very different nuclear charges. The summation is performed over all atom pairs. Atomic triplet intensity  is negligible in most cases. If the molecular intensity is extracted from an experimental pattern by subtracting other contributions, it can be used to match and refine a structural model against the experimental data.
  Similar methods of analysis have also been applied to analyze electron diffraction data from liquids.